Method and apparatus for localized material removal by electrochemical polishing

ABSTRACT

An apparatus for electropolishing a conductive material layer is disclosed. The apparatus comprises a porous conductive member configured to contact the conductive layer and having a first connector for receiving electrical power, an electrode insulatively coupled to the porous conductive member having a second connector configured to receive electrical power, a holder insulatively coupled to the porous conductive member and the electrode configured to establish relative motion between the porous conductive member and the conductive layer, and a power supply coupled to the first connector and the second connector configured to supply the electrical power between the electrode and the porous conductive member for electropolishing the conductive layer.

FIELD

The present invention relates to manufacture of semiconductor integratedcircuits and, more particularly to a method for electrochemically orelectrochemical-mechanically removing unwanted portions of conductivelayers without adversely affecting the wanted portions.

BACKGROUND

Conventional semiconductor devices generally include a semiconductorsubstrate, usually a silicon substrate, and a plurality of sequentiallyformed dielectric interlayers such as silicon dioxide and conductivepaths or interconnects made of conductive materials. Copper and copperalloys have recently received considerable attention as interconnectmaterials because of their superior electromigration and low resistivitycharacteristics. Interconnects are usually formed by filling copper by adeposition process in features or cavities etched into the dielectricinterlayers. The preferred method of copper deposition iselectroplating. In an integrated circuit, multiple levels ofinterconnect networks laterally extend with respect to the substratesurface. Interconnects formed in sequential layers are electricallyconnected using vias or contacts.

In a typical interconnect fabrication process, first an insulatingdielectric layer is formed on the semiconductor substrate. Patterningand etching processes are performed to form features such as trenchesand vias in this insulating layer. Then, copper is electroplated to fillall the features after the deposition of a barrier and seed layer. Afterdeposition and annealing of the copper layer, the excess copper(overburden) and barrier films left outside the cavities defined by thefeatures have to be removed to electrically isolate the conductorswithin the cavities. Processes such as chemical mechanical polishing(CMP), chemical etching, electrochemical etching or polishing, orelectrochemical mechanical etching or polishing techniques may beemployed to remove the overburden copper layer.

This removal process needs to be performed in a highly uniform manner.If there are copper thickness non-uniformities present on the workpieceor if the removal process introduces removal rate non-uniformities, asthe thickness of the overburden conductor such as copper is reduced bythe removal process, residual copper may be left at various locationsover the surface of the wafer. Continuation of the removal process toremove the residual copper regions may cause over-removal of copper fromother regions which have already been freed of overburden copper. Thiscauses copper loss from some of the features surrounding the areascontaining the residual copper. As can be appreciated, such conductorloss from features causes resistance increases and defects and is notacceptable.

FIG. 1A shows an exemplary wafer with a non-uniform copper layer 12 withsurface 14. Although not necessary, the non-uniformity of the copperlayer 12 may be a result of an imperfect polishing or planarizationprocess or a result of the copper deposition step. It is, for example,well-known that copper deposition processes often yield over-plated orsuper-plated regions, especially over the high aspect ratio and densefeatures. In these regions, the thickness of the copper overburden maybe 500–5000 Angstrom or thicker compared to other parts of the wafer.

The copper layer 12 in FIG. 1A is formed on a dielectric layer 15, whichis previously coated with a barrier layer 16. The copper layer 12 fillsfeatures 17 and the trench 18. As illustrated in FIG. 1A, due to thenon-uniformity of the layer, copper layer has thin copper regions 22with thin copper overburden layer and thick copper regions 24 with thickcopper overburden layer. As shown in FIG. 1B, as the copper layer 12 ispolished down using a removal process, thin copper regions 22 arepolished down faster than the thick copper regions 24. As a result,material removal from the thin copper regions 22 is completed fasterthan the thicker copper regions, thus leaving residual copper regions 26on the surface of the substrate. Residual copper region 26 represents avariable-thickness copper overburden (defined as the region between thedashed line and the surface 26A) and it has to be removed. FIG. 2 showsin plan view, an exemplary semiconductor wafer 10 having exemplaryresidual copper regions 26 distributed on the surface of the wafer. Theresidual copper regions 26 form conductive bridges between the featuresright under them.

As shown in FIG. 1C removal of residual copper regions by extending theduration of the traditional removal processes cited above may causemetal loss or dishing at the neighboring features which were previouslyfreed from the copper overburden layer. This is also a common problem inCMP of Cu. Due to within die non-uniformity of Cu layers, there may bethick and thin regions of overburden Cu within a given die. Usuallythick Cu region is over the dense small features. During CMP, thin Curegions clear first. However, to clear the thick Cu regions the wafer isover-polished. During this overpolishing period, the regions which werealready cleared off overburden Cu gets over processed giving rise to thedishing or erosion defects as mentioned above.

SUMMARY

The invention provides a method and an apparatus to electroetch orelectropolish a conductive material layer deposited on a surface of asemiconductor substrate. An apparatus for electropolishing a conductivematerial layer is disclosed. The apparatus comprises a porous conductivemember configured to contact the conductive layer and having a firstconnector for receiving electrical power, an electrode insulativelycoupled to the porous conductive member having a second connectorconfigured to receive electrical power, a holder insulatively coupled tothe porous conductive member and the electrode configured to establishrelative motion between the porous conductive member and the conductivelayer, and a power supply coupled to the first connector and the secondconnector configured to supply the electrical power between theelectrode and the porous conductive member for electropolishing theconductive layer.

In aspects of the invention, the porous conductive member is a brushmade of flexible wires. The flexible wires are made of inert material.

In another aspect of the invention, the porous conductive membercontacts an area of the workpiece that is less than 10% of an area ofthe workpiece.

In yet another aspect of the invention, the electrical power appliedbetween the electrode and the porous conductive member is reduced whenthe conductive layer is substantially removed.

Advantages of the invention include improved control of electropolishedmaterial to improve device consistency and yield.

DRAWINGS

The invention is described in detail with reference to the drawings, inwhich:

FIG. 1A shows an exemplary wafer with a non-uniform copper layer;

FIG. 1B shows a polished copper layer using a typical removal process;

FIG. 1C shows removal of residual copper regions by extending theduration of traditional removal processes;

FIG. 2 shows in plan view, an exemplary semiconductor wafer havingexemplary residual copper regions distributed on the surface of thewafer;

FIG. 3 shows an exemplary electrochemical removal system with anembodiment of remover placed on a surface of a semiconductor wafer inaccordance with the present invention;

FIG. 4 shows the remover in accordance with an embodiment of the presentinvention;

FIGS. 5A–5D illustrate one embodiment of a local electrochemical removalprocess of the present invention;

FIG. 6A shows an exemplary surface of the wafer after completingelectropolishing of copper deposits in accordance with the presentinvention; and

FIG. 6B shows, following barrier layer removal step, a highly planarflat surface without dishing and erosion defects in accordance with thepresent invention.

DETAILED DESCRIPTION

The present invention provides a method and an apparatus to electroetchor electropolish a conductive material layer deposited on a surface of asemiconductor substrate. It should be noted that the technique of thepresent invention may be referred to as electroetching,electropolishing, electrochemical etching, electrochemical polishing,electrochemical mechanical etching, electrochemical mechanical polishingamong many other names. Such processes will be referred to aselectrochemical removal processes for describing the present invention.

One embodiment of the present invention provides a method and apparatusto remove any excess conductive material that is left on a top surface(field) of a dielectric layer on a workpiece such that the conductor,such as copper, remains confined only within features etched into thedielectric layer. One other embodiment of the present invention providesa method to remove all conductive material on the field region of aworkpiece such as a wafer.

In one embodiment, the present invention comprises a conductive memberwhich is adapted to touch a conductive layer to be removed, and anelectrode which is preferably held close to the conductive member, theconductive member being in between the electrode and the conductivelayer to be removed. The conductive layer may be an excess conductivelayer left on certain portions of a dielectric surface of a wafer afteran incomplete removal process or it may be a newly deposited conductivelayer with overburden portion substantially all over the dielectricsurface of a wafer.

The electrode may be physically attached to the assembly of theconductive member, provided that, it is electrically isolated from theconductive member. Alternately other mechanisms may be used to keep theelectrode in proximity of the conductive member. During theelectrochemical removal process, as the conductive layer and theconductive member as well as the electrode are wetted by a processsolution, the conductive layer is contacted by the conductive member. Toinitiate electrochemical removal, a potential difference is appliedbetween the conductive member and the electrode while a relative motionis established between the conductive member and the conductive layer bya moving mechanism.

The electrode is preferably stationary with respect to the conductivemember although it is possible to impart oscillatory or rotationalmotions to the electrode during the process. The conductive member ismade of an electrically conductive porous material and has the abilityto allow the process solution touch the conductive layer. A preferreddesign of the conductive member is a conductive brush. Material removalrate during the process may be controlled by adjusting the distancebetween the conductive member and the electrode as well as by adjustingthe voltage applied between (or current passing through) the conductivemember and the electrode. Selection of the process solution is alsoimportant for material removal rate.

FIG. 3 shows an exemplary electrochemical removal system 100 with anembodiment of remover 102 placed on a surface 104 of a semiconductorwafer 106. As will be described below. The surface of the wafer 106includes a conductive film or remainder of a film that is left from aprevious pianarization or material removal process. In this embodimentthe conductive film is a copper film; however, it may be any other metalsuch as Cu alloys, Ni, Pb, Fe, magnetic alloys, Ag, Cr, Au etc. Aprocess solution 108 contacts the surface 104 and the components of theremover 102. An exemplary process solution may be a phosphoric acidsolution, a very dilute (<10%) sulfuric acid solution or a saltsolution. The wafer 106 is retained and moved by a wafer carrier 110.The carrier 110 may rotate or move the wafer laterally or verticallyusing a moving mechanism (not shown).

The remover 102 is held by a holder 112, which may place the remover onselected locations on the surface 104 and move the remover 102 overdifferent locations on the surface 104 during the process. The holder112 may be a robotic arm controlled by a computer system (not shown).The computer system may drive the remover over the previously detectedregions that have left over copper films on the surface or the removercan be scanned over the whole surface of the wafer. In an alternativeembodiment, the holder and the remover are movably attached so that theremover can be angled in different directions. The remover comprises aconductive member 114, which touches the surface 104 during the process,and an electrode 116 that is isolated from the conductive member 114.The conductive member is a porous and conductive contact member to touchand sweep the conductive material on the wafer surface. The conductivemember 114 and the electrode are connected to a positive terminal andnegative terminal of a power supply 118, respectively. It should benoted that although description here is for a movable remover, it ispossible that the remover is stationary and the wafer surface is movedby the wafer carrier 110 to allow the remover scan the selected areas ofthe wafer or substantially the whole surface of the wafer.

FIG. 4 shows, in detail, the remover 102 of the present embodiment. Asshown in FIG. 4, the conductive member 114 is electrically insulatedfrom the electrode 116 and the holder 112 by insulation layers 120. Theelectrode 116 can be a metallic plate such as a copper plate. Althoughin this embodiment, it is rectangular, the shape of the electrode may beany geometrical form. The conductive member is a porous structure thatthe process solution can flow through it. In this embodiment, theconductive member may be a conductive brush having multiple flexibleconductive elements 122 such as fine conductive wires of metals, alloysor polymers. The conductive elements may be made of conductive materialsthat do not chemically react with the process solution. Inert metalssuch as Pt, Ir, Pd and alloys containing such inert materials can beused for this purpose. Conductive polymers are also examples of suchinert materials. As will be described later, the conductive members donot have to be made of inert materials. Sacrificial conductive membersmade of materials such as copper may also be used. In this case, suchconductive members may have to be replaced after certain period of use.

Conductive members are selected from the materials that are flexible inmacro-scale and rigid in micro-scale. The conductive elements may beattached to a base 123 which may be used to apply electricity to theconductive elements. As will be described below, during the process whenthe conductive member 114 is placed on the surface of the wafer, theconductive elements flex towards the electrode and establish a gap 124between the electrode and the conductive member. At this point, anapproximate operation distance ‘d’, or gap, between the conductivemember and the electrode may be in the range of 2–20 millimeters (mm).An exemplary process voltage range may be less than 10 volts, preferablyless than 5 volts.

FIGS. 5A–5D illustrate one embodiment of the local electrochemicalremoval process of the present invention using the remover 102. In thisexample, remover 102 is used to remove the residual copper film 130 fromthe surface 104 of wafer 106. As shown in the FIGS. 5A–5D, the residualcopper film 130 is a top part of a copper layer 132 that fills features134A, 134B, 134C and 134D in a dielectric layer 135 that is formed onthe wafer 106. The feature 134A may be a via or narrow trench and thefeatures 134B and 134C may be narrow trenches. The feature 134D may be awider trench. Although in this example, the features 134A–134D haveshown with small, medium and large width, and are placed in certainorder on the wafer, this is for the purpose of clarity and to describethe invention. Accordingly, width, dimension and order placement of thefeatures 134A–134D may vary on the wafer 106 and it is within the scopeof the present invention.

In this example, the residual copper film 130 comprises a thin region133A, which is generally located over the features 134A–134B, and athick region, which is generally located on the features 134C and 134D.Between the dielectric 135 and the copper layer 132, a barrier layer 137such as a Ta/TaN layer may also be located. The barrier layer 137 coatsthe features 134A–134D and the surface 136 of the dielectric layer 135.Aim of the process of the present invention is removing the residualcopper film 130 from the surface 136 of the dielectric layer withoutcausing excessive copper removal from the copper filled features134A–134D irrespective of the thickness of the copper overburden overthe features.

As shown in FIG. 5A, during electrochemical material removal, theconductive member 114 is placed on surface 139 of the residual copperfilm 130 and removal potential is applied between the conductive member114 and the electrode 116 while a relative motion is maintained betweenthe wafer 106 and the conductive member 114. As the wafer 106 is moved,conductive elements 122 slide over the surface 139 of the residualcopper film 130 and establish electrical contact with the film. Duringthe electrochemical removal process of the present invention, theconductive elements are laid on the copper film 130 and slide on thesurface 139 with applied relative motion. The conductive elements 122are positioned on the surface 139 of the copper film lengthwise so thata portion (for example more than half length) of the conductive memberfully physically contacts the film to be removed. Voltage appliedbetween the conductive member 114 and the electrode 116 renders theconductive member 114 and the conductive elements 122 anodic. Therefore,a first current flows between the conductive elements 122 and theelectrode 116.

If the conductive elements 122 of the conductive member 114 are made ofan inert material, that cannot be etched by the electrochemical processin the process solution, the applied potential can be selected tominimize this first current, which will be referred to as “leakagecurrent” hereinafter. Because of the applied voltage, some amount of gasgeneration may occur depending upon the applied voltage if the processsolution is aqueous. Although the applied voltage causes a leakagecurrent between the electrode 116 and the conductive elements 122, italso causes a current to pass between the residual copper film 130 andthe electrode 116 once the conductive elements 122 make physical contactwith the surface of the residual copper film. Conductive elements 122 ofthe conductive member allow the process solution 108 to make contact tothe surface 139 of the residual copper film 130 since they have pores oropenings between them. The conductive elements 122 form a porousconductive medium through which the process solution 108 as well as aprocess current can pass. Other designs of conductive elements may beused to practice this invention as long as they have this stated porousnature.

Consequently, when the conductive elements 122 touch the surface 139 andrender the residual copper film anodic, electrochemical reaction cantake place between the process solution 108 and the surface of theresidual copper film. The process solution 108 is selected such thatunder a given anodic potential, electrochemical removal of copper in theprocess solution is more efficient than the electrochemical removal of amaterial from the conductive elements 122. As stated before, if inertmaterials are used to construct the conductive porous member, nomaterial may be removed from the conductive porous member during theprocess.

When the conductive elements 122 are touched to the surface 139 and ananodic potential is applied to the elements the surface 139 of theresidual copper film is rendered anodic and electrochemical dissolutiontakes place from the surface. A process current passes through thecircuit in addition to the leakage current. This process current may behigher than the leakage current at the selected operational voltagesince copper can be readily removed by an electrochemical reaction inthe process solution and is deposited on the electrode after passingthrough the pores in between the conductive elements.

FIG. 5B shows an instant during the removal of the residual copper film130. As shown in FIG. 5B, as the material removal from the residualcopper film 130 is continued, the thin region 133A (see FIG. 5A) of thefilm 130 is removed from the top of the features 134A and 134B, whichleaves copper deposits 138A and 138B confined in the features 134A and134B. Such area having features with copper deposits only confined inthe features will be referred to as residue-free area. At this stage,although thinner, the originally thick region 133B of the residual film130 still electrically shorts the top of the features 134C and 134D. Theremoval of the residual film above the features 134A and 134B results inexposing a portion of the barrier layer 137 on the surface 136 of thedielectric layer 135.

During the removal, top surface 141 of the copper deposits 138A and 138Bmay be slightly etched to form recesses 142 on top of the deposits.Therefore, physical contact between the top surfaces 141 of the depositsand the conductive elements 122, which are substantially parallel to thetop surfaces 141 of the deposits, is lost. Process solution fills thegap between the conductive members 122 and the top surface of thedeposits 138A and 138B. This situation can be seen in FIG. 5C whichexemplifies location of one of the conductive members 122 as it ispassed over the deposit 138A in an instant of the process. Theconductive member 122 is on the barrier layer 137 and is separated fromthe top surface 141 of deposit 138A with a gap 140 filled with processsolution 108.

Referring back to FIG. 5B, in the residue-free area the conductiveelements 122 slide over the barrier layer and do not contact the copperdeposits 138A and 138B for the reasons explained above. High resistivityof the process solution 108 also hinders an electropolishing currentflow between the conductive elements 122 and the top of the copperdeposits 138A and 138B. In the residue-free area, copper deposits 138Aand 138B are physically separated from one other, but not electrically.The barrier layer 137 on the surface 139 still connects themelectrically. However, in comparison to the copper, barrier layermaterial, for example Ti, W, WN, WCN, Ta or TaN, has a significantlyhigher electrical resistivity. Because of the better conductivityprovided by the remaining residual copper, at this stage,electrochemical material removal selectively continues on the surface ofthe remaining portion of the residual copper as the conductive elements122 touch the remaining residual copper film. Electropolishing currentflows with less resistivity in the remaining residual copper film.

It will be appreciated that although the relative motion between thewafer and the conductive member allows conductive elements 122 to sweepthe residue-free area and the neighboring portions of the barrier layer137, due to the better conductivity, electrochemical removal selectivelyproceeds on the remaining residual copper films when the conductiveelements sweep such remaining films. The high electrical resistivity ofthe exposed barrier layer portions and also the high electricalresistivity of the process solution hinder the flow of electrochemicalremoval current and hence hinder the removal of the wanted copper in thefeatures after the removal of the unwanted residual copper in suchareas.

FIG. 5D illustrates surface 104 of the wafer 106 as the rest of theresidual copper film 130 is being removed by the conductive members 122and size of the residue-free area is expanded. At this stage copperdeposits 138A, 138B, 138C and 138D are physically separated from oneanother with portions of the barrier layer 137 and are confined in thefeatures 134A, 134B, 134C and 134D, respectively. Once the residual filmis removed by the conductive members 122, as described above inconnection with FIG. 5B, in the residue-free area, high electricalresistivity of the exposed barrier layer and the process solutionsignificantly reduces the current flow and the material removal from thefeatures. As opposed to prior art processes, in the process of thepresent invention, dishing is arrested in the features 134A and 134B,although the residual film is removed above them before the features134C and 134D. As shown in FIGS. 1B and 1C in the prior art, as theresidual copper is removed, removal process causes excessive dishing inthe neighboring features.

In one alternative embodiment, the conductive elements 122 may be madeof the same material to be removed from the substrate surface, i.e.copper for copper removal. In this case, during electrochemical removalprocess both the copper on the wafer and the conductive elements areetched. Electroetching of the copper conductive elements together withthe top surfaces of the copper deposits may contribute formation of athicker boundary layer or salt layer on the deposits. This salt layercontains a viscous solution of copper phosphates if the process solutioncontains phosphoric acid. For example, when the salt layer forms in thegap 140 shown in FIG. 5C, it further slows down material removal ratefrom the top surface 141 of the deposit 138A. It should be noted thatsalt layers are high resistivity layers that form on the conductivesurfaces during electropolishing of such surfaces, and are well known inthe art of electropolishing. As such layers get thicker, materialremoval from the conductive surfaces is reduced.

FIG. 6A shows the exemplary surface 104 of the wafer 106 aftercompleting the electropolishing of copper deposits 138A–138D in thefeatures 134A–134D. As shown in FIG. 6A the top surfaces of the copperdeposits may be lowered up to the level of the barrier layer duringmaterial removal step. It is, of course, beneficial to minimize copperrecess shown in FIG. 6A. As shown in FIG. 6B, following barrier layerremoval step, a highly planar flat surface is obtained without dishingand erosion defects.

The above described localized material removal process (LMRP) may beapplied after various material removal or material deposition methods.For example, LMRP may be applied after chemical mechanical polishing(CMP) to clear the final remnants of copper from the surface.Alternatively, LMPR can be applied after a sequence of processes such aselectrochemical deposition (ECD) followed by electrochemical mechanicalpolishing (ECMP) and as a final step LMPR to remove residual conductors.Another example may be ECD followed by electrochemical mechanicaldeposition (ECMD), which is followed by an electrochemical polishing(ECP). In this process sequence, a LMPR step may be used to removeresidual copper. The LMPR method may also be applied after ECD or ECMDprocesses, or a process using both by beginning with ECD, which isfollowed by ECMD.

ECMD process produces a planar copper layer on a wafer and descriptionsof various ECMD methods and apparatus can be found in the followingpatents and pending applications, all commonly owned by the assignee ofthe present invention. U.S. Pat. No. 6,176,992 entitled “Method andApparatus for Electrochemical Mechanical Deposition,” U.S. Pat. No.6,354,116 entitled “Plating Method and Apparatus that Creates aDifferential Between Additive Disposed on a Top Surface and a CavitySurface of a Workpiece Using an External Influence,” U.S. Pat. No.6,471,847 entitled “Method for Forming Electrical Contact with aSemiconductor Substrate” and U.S. Pat. No. 6,610,190 entitled “Methodand Apparatus for Electrodeposition of Uniform Film with Minimal EdgeExclusion on Substrate.”

Although the present invention has been particularly described withreference to the preferred embodiments, it should be readily apparent tothose of ordinary skill in the art that changes and modifications in theform and details may be made without departing from the spirit and scopeof the invention.

1. A process for electrochemically removing overburden conductivematerial formed over cavities having cavity conductive material thereinon a surface of a workpiece, comprising the steps: contacting theoverburden conductive material with a remover including a porousconductive member insulatively coupled to an electrode, the removerbeing smaller in area than the workpiece; applying a voltage between theporous conductive member and the electrode; establishing relative motionbetween the workpiece and the remover; and electrochemically removingthe overburden conductive material on the surface of the workpiece whileestablishing relative motion.
 2. The process of claim 1, wherein thestep of contacting includes contacting less than 10% of an area of theworkpiece surface.
 3. The process of claim 1 further comprisingmaintaining a gap between the electrode and the porous conductivemember.
 4. The process of claim 3 further comprising bridging the gapbetween the electrode and the porous conductive member.
 5. The processof claim 3, wherein the gap is in the range of 0.1 to 5 millimeters. 6.The process of claim 1, wherein the step of contacting the overburdenconductive material includes laying an area of the porous conductivemember on the overburden conductive material.
 7. The process of claim 1,wherein the step of establishing relative motion includes sweeping theporous conductive member across the overburden conductive material. 8.The process of claim 1, wherein the step of establishing relative motionincludes sweeping the porous conductive member across substantially anentire surface of the workpiece.
 9. The process of claim 1, wherein thestep of establishing relative motion includes moving the surface of theoverburden conductive material to sweep the porous conductive memberacross the overburden conductive material.
 10. A process forelectrochemically removing overburden conductive material formed overcavities having cavity conductive material therein on a surface of aworkpiece comprising the steps: contacting the overburden conductivematerial with a porous conductive member insulatively coupled to anelectrode; applying a voltage between the porous conductive member andthe electrode; establishing relative motion between the porousconductive member insulatively coupled to the electrode and theworkpiece to electrochemically remove the overburden conductive materialon the surface of the workpiece; and self-limiting the electrochemicalremoval of the overburden conductive material after exposing the cavityconductive material.
 11. The process of claim 10, wherein the step ofself-limiting includes contacting the porous conductive member with anunderlying barrier layer.
 12. The process of claim 11, wherein the stepof self-limiting includes sensing a reduced current drop between theporous conductive member and the electrode.
 13. The process of claim 1,wherein the porous conductive member comprises a conductive brush. 14.The process of claim 1, wherein the step of contacting includes flexingthe porous conductive member towards the electrode to define a gaptherebetween.
 15. The process of claim 1, wherein the step ofelectrochemically removing overburden conductive material is localizedto the area of the remover.
 16. The process of claim 14, wherein thestep of removing comprises controlling a removal rate by adjusting thegap.
 17. The process of claim 14, wherein the gap is between about 2 and20 millimeters.
 18. The process of claim 1, wherein the step ofestablishing relative motion includes sweeping the porous conductivemember across selected areas of the workpiece.
 19. The process of claim1, wherein the voltage is less than about 20 volts.
 20. The process ofclaim 1, wherein the voltage is less than about 5 volts.
 21. The processof claim 1, further comprising providing a process solution between theporous conductive member and the electrode.
 22. The process of claim 21,wherein the porous conductive member comprises conductive material thatdoes not chemically react with the process solution.
 23. The process ofclaim 22, wherein the conductive material that does not chemically reactwith the process solution comprises at least one of platinum, iridium,palladium, alloys thereof, and polymers.
 24. The process of claim 21,wherein the porous conductive member comprises conductive material thatchemically reacts with the process solution.
 25. The process of claim24, wherein the conductive material that chemically reacts with theprocess solution comprises the same material as the overburdenconductive material.
 26. The process of claim 25, wherein the step ofremoving includes etching the porous conductive material and theoverburden conductive material.
 27. The process of claim 24, wherein theconductive material that chemically reacts with the process solutioncomprises copper.
 28. The process of claim 13, wherein the conductivebrush comprises a plurality of multiple flexible conductive elements.29. The process of claim 28, wherein the plurality of flexibleconductive elements is flexible in a macro scale and rigid in a microscale.
 30. The process of claim 28, wherein the step of contactingcomprises positioning the plurality of flexible conductive elements onthe workpiece such that a portion of the porous conductive materialcontacts the overburden conductive material.
 31. The process of claim30, wherein the portion comprises more than about half the length of theflexible conductive elements.